Back in August, before the GOP establishment's nightmare was getting saddled with Donald Trump as their presidential nominee, their nightmare was a Donald Trump third-party bid.
Even if a Trump candidacy only drew 4 or 5 percent of the national vote, it would pose a massive problem for the Republican nominee, since Trump supporters would almost certainly come overwhelmingly from white voters without a college degree — a very Republican-leading demographic. That's why getting him to sign a "loyalty pledge" back in September committing him to support the eventual nominee, whoever it was, was considered a big victory at the time.
But now Trump is warning that he might alter the deal.
The context is a story published late last week indicating that veteran Republican communications operative Liz Mair is seeking to create an anti-Trump fundraising vehicle that would allow backers of various candidates to come together even while Rubio, Jeb, Christie, Cruz, etc. continue to run against each other. Trump's contention is that this form of collusion would be "unfair" and would compromise his commitment to the unity pledge.
This is the GOP establishment's basic bind. To beat Trump, they need to work together. But to beat Hillary Clinton, they need Trump's fans to vote GOP in 2016. And any excessively "unfair" hits on Trump risk jeopardizing that.